Calorie deficit calculator

How your budget is worked out

It's the same calculation Weight Loss Tracking Buddy makes when you set a goal, in four steps.

1. Your body at rest. Your basal metabolic rate (BMR), from the revised Harris-Benedict equations, which take your weight in kilograms and height in centimetres:
   • Men: 88.362 + 13.397 × weight + 4.799 × height − 5.677 × age
   • Women: 447.593 + 9.247 × weight + 3.098 × height − 4.330 × age
2. Your day. Your BMR times your activity level: 1.2 if you're not very active, 1.375 lightly active, 1.55 active and 1.725 very active. That's roughly what holds your weight where it is.
3. Your pace. A pound of body weight is taken as 3,500 kcal, so losing 1 lb a week means eating about 500 kcal a day less than you use, and gaining it means 500 more.
4. The floor. The budget never goes below 1,200 kcal a day. If your pace would take it lower, the budget stays at 1,200 and your goal date moves later instead.

An estimate, not a prescription

The equations describe the average person of your size, age and sex. Your own needs can be a few hundred calories either side, and they fall a little as you lose weight. Researchers who modelled this found the 3,500 kcal rule overstates how much you lose over months and years (Hall and colleagues, 2011), so the further off your goal, the more your real date may drift.

The best check is your own trend. Follow the budget for two or three weeks, then compare how fast your weight is really moving with the pace you chose. The NHS puts an average man's needs at around 2,500 kcal a day and an average woman's at around 2,000, which gives you a sense of scale.

If you're pregnant or breastfeeding, under 18, have had an eating disorder or have a medical condition, talk to a doctor before changing what you eat.

Turning a budget into a habit

Knowing your number is the easy part. In the app, your budget comes from your goal, and with Pro you can snap a meal or scan a label to see what's left of it, with your macros as rings. With Apple Health, the energy you burn moving about is added to that day's budget too.

Sources

• Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. The American Journal of Clinical Nutrition, 1984; 40(1): 168 to 182.
• Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011; 378(9793): 826 to 837.
• NHS, Understanding calories.